One of the standard designs of light emitting diodes utilizes a polymer material filled with fluorescent phosphor molded over the blue-light-emitting element (a so-called phosphor-conversion LED or pc-LED). Such design allows efficient mass production of LEDs with low variability in light emission. For this design, a mixture of fluorescent phosphor powders is used to achieve the required spectrum of light. For the polymer matrix, silicone is the first material of choice due to its high optical clarity, thermal stability, and easy processability. A significant drawback of the phosphor-filled polymer design is inefficient thermal transfer from the luminescent particles to the outside environment. Heat produced as a result of the conversion of the shorter-wavelength blue light into a longer-wavelength light, such as yellow light, must be efficiently removed; otherwise the increased operating temperature of the LED will result in reduced light efficacy and premature degradation of the luminescent material.
Heat transfer from the phosphor particles occurs through the layer of the surrounding polymeric matrix, which in general has poor thermal transfer properties. The thermal conductivity of silicones varies between 0.12 and 0.20 watts per meter Kelvin (W/mK) with the overall range for polymers being about 0.1-0.5 W/mK. The most thermally conductive polymeric materials are semicrystalline polymers, which are not suitable for LED encapsulation due to a high degree of light scattering from crystallites. Multiple solutions have been developed to increase thermal conductivity of polymers by the incorporation of metal or ceramic powders. However, due to a large size of the filler particles in conventional blends, such composites are not optically transparent.
Furthermore, in addition to thermal conductivity, the need for flame retardant optical grade polymers is increasing with the rapid expansion of light emitting diodes into the general lighting area. According to the ANSI/UL 8750 standard for LED lighting products, the enclosure (external parts) of the fixture must be designed in a way to ensure protection of the neighboring construction from fire that may result from ignition within the fixture. If polymers are used for the enclosure, a flammability rating of V-1 is required for the LED lenses and 5-VA for all other parts (ANSI/UL 94). Among the optical grade plastics on the market, only special flame retardant polycarbonates meet these criteria. Silicones offer a number of advantages compared to polycarbonates in terms of blending with phosphors and fabricating of components. Therefore, adding flame retardancy to the set of other properties will allow using silicones as encapsulants without having to build an additional protecting enclosure around the LED module.
Typically the flame retardancy of silicones may be improved by incorporation of various inorganic fillers, such as mica, titanium dioxide, carbon black, calcium carbonate, or diatomaceous earth. These fillers reduce the time to ignition as well as act as sources of char to insulate the combustion area from the adjacent areas of material. A significant drawback of the inorganic fillers is the loss of transparency resulting from their addition; therefore they only can be used in the products for which transparency is not important. Another possibility includes the use of salts of platinum and rhodium and oxides of rare earth metals. These additives can improve flame resistance to some extent without compromising transparency. However, their high cost discourages their use in large amounts.
Fumed silica is also known as a filler for silicones, primarily for increasing viscosity and tensile strength and for cost reduction. The surface of the silica particles is typically made hydrophobic by hexamethyldisilazane to improve compatibility with silicones. However, this approach does not provide sufficient compatibility as some loss of transmitted light is possible due to scattering from large aggregated particles.
It has been reported in the literature that star polymers comprised of silica nanoparticles grafted with monoglycidylether-terminated poly(dimethylsiloxane) (PDMS-G) showed enhanced thermal stability. Liu et al., Poly(dimethylsiloxane) Star Polymers Having Nanosized Silica Cores, Macromol. Rapid Commun., 25 (2004) 1392-1395. However, it was noted that steric hinderance limited the amount of PDMS-G that was capable of reacting with the silica surface. Moreover, the PDMS-G was terminated with a non-reactive n-butyl group, which would have the disadvantage of limiting crosslinking and densification of the polymer shell.
Thus, it would be an advantage to provide light transmissive silicone materials having increased thermal conductivity or fire retardancy for use with LEDs.